Editing Swarm behaviour (section)

====Bird migration====
[[File:PSM V84 D217 2 Flocking habit of migratory birds fig5.jpg|thumb|right|Large bird typically migrate in V [[echelon formation]]s. There are significant aerodynamic gains. All birds can see ahead, and towards one side, making a good arrangement for protection. ]]
{{Main|Bird migration}}
{{See also|Reverse migration (birds)}}

Approximately 1800 of the world's 10,000 bird species are long-distance migrants.<ref>{{cite journal |author=Sekercioglu, C.H.|year=2007|title=Conservation ecology: area trumps mobility in fragment bird extinctions|journal=Current Biology |volume=17|pages=R283–R286|doi=10.1016/j.cub.2007.02.019|pmid=17437705|issue=8|s2cid=744140|doi-access=free|bibcode=2007CBio...17.R283S }}</ref> The primary motivation for migration appears to be food; for example, some hummingbirds choose not to migrate if fed through the winter. Also, the longer days of the northern summer provide extended time for [[breeding in the wild|breeding]] birds to feed their young. This helps [[diurnality|diurnal]] birds to produce larger [[clutch (eggs)|clutch]]es than related non-migratory species that remain in the tropics. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season. These advantages offset the high stress, physical exertion costs, and other risks of the migration such as predation.

Many birds migrate in flocks. For larger birds, it is assumed that flying in flocks reduces energy costs. The V formation is often supposed to boost the efficiency and range of flying birds, particularly over long [[bird migration|migratory]] routes. All the birds except the first fly in the [[upwash]] from one of the [[wingtip vortices]] of the bird ahead. The upwash assists each bird in supporting its own weight in flight, in the same way a [[glider aircraft|glider]] can climb or maintain height indefinitely in rising air. Geese flying in a [[V formation]] save energy by flying in the updraft of the wingtip vortex generated by the previous animal in the formation. Thus, the birds flying behind do not need to work as hard to achieve lift. Studies show that birds in a V formation place themselves roughly at the optimum distance predicted by simple aerodynamic theory.<ref>[https://archive.today/20090521201002/http://stinet.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA401264 Drag Reduction from Formation Flight. Flying Aircraft in Bird-Like Formations Could Significantly Increase Range]; Defense Technical Information Center; April 2002; Retrieved February 27, 2008</ref> Geese in a V-formation may conserve 12–20% of the energy they would need to fly alone.<ref>{{cite journal |author1=Hummel D. |author2=Beukenberg M. |year= 1989 |title= Aerodynamische Interferenzeffekte beim Formationsfl ug von Vogeln |journal= J. Ornithol. |volume= 130 |issue= 1 |pages= 15–24 |doi= 10.1007/BF01647158|bibcode=1989JOrn..130...15H |s2cid=823269 }}</ref><ref>{{cite journal |author1=Cutts, C. J. |author2=J R Speakman |name-list-style=amp |year=1994|title=Energy savings in formation flight of Pink-footed Geese|journal= J. Exp. Biol.|volume= 189|pages=251–261 |url=http://jeb.biologists.org/cgi/reprint/189/1/251.pdf |pmid=9317742|issue=1|doi=10.1242/jeb.189.1.251 |bibcode=1994JExpB.189..251C }}</ref> [[Red knot]]s and [[dunlin]]s were found in radar studies to fly 5&nbsp;km per hour faster in flocks than when they were flying alone.<ref name=newton>{{cite book |author=Newton, I. |title=The Migration Ecology of Birds |publisher=Elselvier |year=2008 |isbn=978-0-12-517367-4}}</ref> The birds flying at the tips and at the front are rotated in a timely cyclical fashion to spread flight [[fatigue (medical)|fatigue]] equally among the flock members. The formation also makes communication easier and allows the birds to maintain visual contact with each other.

[[File:Flock of birds at Rome.ogg|thumb|left|Common starlings]]

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|video1=[http://www.red3d.com/cwr/boids/ Lobster Migration scene]<br/><small>– from [[The Trials of Life]]</small>
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Other animals may use similar drafting techniques when migrating. [[Lobster]]s, for example, migrate in close single-file formation "lobster trains", sometimes for hundreds of miles.

The Mediterranean and other seas present a major obstacle to soaring birds, which must cross at the narrowest points. Massive numbers of large [[bird of prey|raptor]]s and storks pass through areas such as [[Gibraltar]], [[Falsterbo]], and the [[Bosphorus]] at migration times. More common species, such as the [[European honey buzzard]], can be counted in hundreds of thousands in autumn. Other barriers, such as mountain ranges, can also cause funnelling, particularly of large diurnal migrants. This is a notable factor in the [[Central America]]n migratory bottleneck. This concentration of birds during migration can put species at risk. Some spectacular migrants have already gone extinct, the most notable being the [[passenger pigeon]]. During migration the flocks were a mile (1.6&nbsp;km) wide and 300 miles (500&nbsp;km) long, taking several days to pass and containing up to a billion birds.
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